LasR Receptor for Detection of Long-Chain Quorum-Sensing Signals: Identification of N-Acyl-homoserine Lactones Encoded by the avsI Locus of Agrobacterium vitis

Nutrient Requirements Shape the Preferential Habitat of Allorhizobium vitis VAR03-1, a Commensal Bacterium, in the Rhizosphere of Arabidopsis thaliana

A diverse range of commensal bacteria inhabit the rhizosphere, influencing host plant growth and responses to biotic and abiotic stresses. While root-released nutrients can define soil microbial habitats, the bacterial factors involved in plant-microbe interactions are not well characterized. In this study, we investigated the colonization patterns of two plant disease biocontrol agents, Allorhizobium vitis VAR03-1 and Pseudomonas protegens Cab57, in the rhizosphere of Arabidopsis thaliana using Murashige and Skoog (MS) agar medium. VAR03-1 formed colonies even at a distance from the roots, preferentially in the upper part, while Cab57 colonized only the root surface. The addition of sucrose to the agar medium resulted in excessive proliferation of VAR03-1, similar to its pattern without sucrose, whereas Cab57 formed colonies only near the root surface. Overgrowth of both bacterial strains upon nutrient supplementation inhibited host growth, independent of plant immune responses. This inhibition was reduced in the VAR03-1 ΔrecA mutant, which exhibited increased biofilm formation, suggesting that some activities associated with the free-living lifestyle rather than the sessile lifestyle may be detrimental to host growth. VAR03-1 grew in liquid MS medium with sucrose alone, while Cab57 required both sucrose and organic acids. Supplementation of sugars and organic acids allowed both bacterial strains to grow near and away from Arabidopsis roots in MS agar. These results suggest that nutrient requirements for bacterial growth may determine their growth habitats in the rhizosphere, with nutrients released in root exudates potentially acting as a limiting factor in harnessing microbiota.

Potential role of quorum quenching activity of silver nanoparticles in controlling non-filamentous bulking within activated sludge process

The effective prevention and control of non-filamentous bulking is a significant challenge. In this study, the underlying effect of quorum sensing (QS) on inducing non-filamentous bulking and the maintenance effect of silver nanoparticles (AgNPs) on sludge floc stability, aggregation and settleability based on the quorum quenching (QQ) activity during non-filamentous bulking were investigated. The results showed that the concentration of N-acyl homoserine lactone (AHL) increased significantly in the activated sludge system at a high organic load rate (OLR), triggering the AHL-mediated QS. Additionally, the triggered QS promoted exopolysaccharide secretion, reducing the surface charge and hydrophobicity of the sludge aggregates, and further deteriorating the settleability of the sludge aggregates. AgNPs, a quorum sensing inhibitor (QSI), inhibited the AHL-QS based on QQ activity under high OLR, which maintained the physicochemical properties of extracellular polymeric substances (EPS). AgNPs-QQ maintained the surface energy barrier and electrostatic barrier of sludge aggregates and the gel properties of exopolysaccharides, which is favorable for microbial aggregation. The appropriate concentrations of AgNPs (≤10 mg/L) had no negative effect on biological nutrient removal in the sequencing batch reactors (SBRs) at the high organic loading. Therefore, AgNPs effectively prevent and control non-filamentous bulking by their QQ activity in the activated sludge process. Thus, the present study provided new insights into controlling non-filamentous bulking during the activated sludge process.

Different Concentrations of Lactobacillus acidophilus Cell Free Filtrate Have Differing Anti-Biofilm and Immunomodulatory Effects

Probiotics such as various strains of Lactobacillaceae have been shown to have antimicrobial and immunomodulatory activity. In vitro studies have shown that Lactobacilli can decrease bacterial biofilm formation. Effects on immune cells have been unclear with most studies showing anti-inflammatory activity. The mechanism of effects has not been clearly elucidated. In these studies, we used different concentrations of live Lactobacillus acidophilus as well as cell free filtrate (CFF) derived from different concentrations of bacteria. Use of CFF is advantageous as a therapeutic because in vivo it can directly contact immune cells and its concentration is fixed. Both live cells and CFF inhibited Pseudomonas aeruginosa biofilm formation. Importantly, we show that high concentration CFF destroyed mature biofilm. This activity was not due to a lowered pH per se, as pH matched HCl did not remove mature biofilm. High concentration CFF totally inhibited P. aeruginosa growth and was bactericidal (>99.99%), but low concentration CFF was not bactericidal. To examine the immunomodulatory effects of L. acidophilus, we incubated THP-1 monocytes and derived macrophages with CFF and measured TNFα production. CFF did not significantly increase TNFα production in THP-1 monocytes. When cells were prestimulated with LPS, high concentration CFF increased TNFα production even further. In macrophages, high concentration CFF alone increased TNFα production but did not affect LPS prestimulated cells. In contrast, low concentration CFF decreased TNFα production in LPS prestimulated cells. To elucidate the possible mechanisms for these effects, we repeated the experiments using a NF-κB reporter THP-1 cell line. High concentration CFF increased NF-κB activity in monocytes and macrophages. In LPS prestimulated macrophages, only low concentration CFF reduced NF-κB activity. These results suggest that high concentration CFF alone induced NF-κB expression which could account partially for an increase in TNFα production. On the other hand, in macrophages, the lower non-bactericidal concentration of CFF reduced NF-κB expression and decreased TNFα production after LPS prestimulation. Taken together, the results provide evidence that different concentrations of L. acidophilus CFF possess varying bactericidal, anti-biofilm and immunomodulatory effects. This is important in vivo to evaluate the possible use of L. acidophilus CFF in different conditions.

A Whole-Cell Biosensor for Point-of-Care Detection of Waterborne Bacterial Pathogens

Water contamination by pathogenic bacteria is a major public health concern globally. Monitoring bacterial contamination in water is critically important to protect human health, but this remains a critical challenge. Engineered whole-cell biosensors created through synthetic biology hold great promise for rapid and cost-effective detection of waterborne pathogens. In this study, we created a novel whole-cell biosensor to detect water contamination by Pseudomonas aeruginosa and Burkholderia pseudomallei, which are two critical bacterial pathogens and are recognized as common causative agents for waterborne diseases. The biosensor detects the target bacterial pathogens by responding to the relevant quorum sensing signal molecules. Particularly, this study constructed and characterized the biosensor on the basis of the QscR quorum sensing signal system for the first time. We first designed and constructed a QscR on the basis of the sensing module in the E. coli host cell and integrated the QscR sensing module with a reporting module that expressed an enhanced green fluorescent protein (EGFP). The results demonstrated that the biosensor had high sensitivity in response to the quorum sensing signals of the target bacterial pathogens. We further engineered a biosensor that expressed a red pigment lycopene in the reporting module to produce a visible signal readout for the pathogen detection. Additionally, we investigated the feasibility of a paper-based assay by immobilizing the lycopene-based whole-cell biosensor on paper with the aim to build a prototype for developing portable detection devices. The biosensor would provide a simple and inexpensive alternative for timely and point-of-care detection of water contamination and protect human health.

Candidates of quorum sensing bacteria in activated sludge associated with N-acyl homoserine lactones

This study aimed to explore candidates of microbial groups which is associated with quorum sensing in activated sludge. Activated sludge samples were collected from three wastewater treatment plants (WWTP) to analyze N-acyl homoserine lactone (AHL) by Fourier-transform mass spectrometry (FTMS) and 16S rRNA-based microbial community. Among activated sludge samples taken at 3 WWTPs in different seasons, 2 AHL species of N-3-hydroxyoctanoyl-l-homoserine lactone and N-3-hydroxydecanoyl-l-homoserine lactone were detected in the range of ranged of 0.1 ng/L to 1.6 ng/L. The detected AHL species were not dependent on treatment systems nor seasons. From microbial community analysis, population abundance of one strain in Verrucomicrobia and two strains in Holophagaceae had high correlation with AHL concentration in activated sludge. Comamonadaceae had also moderately correlated population with AHL concentrations among quorum sensing bacteria reported previously.

Pseudomonas aeruginosa LasR·DNA Binding Is Directly Inhibited by Quorum Sensing Antagonists

Inhibition of quorum sensing in Pseudomonas aeruginosa is of interest as a possible antivirulence strategy for this pathogenic bacterium. The LasR regulator protein is important in coordinating gene expression in response to quorum sensing signaling molecules. One predominant strategy for LasR inhibition is the development of small-molecule antagonists that mimic the native autoinducer, though the mechanism by which they inactivate LasR is not known. This work reveals that multiple antagonists function by binding to and stabilizing LasR in a conformation that renders it unable to bind DNA. Further analysis of purified LasR complexed with known antagonists indicates that DNA binding can be recovered with the addition of native autoinducer, providing insights into the reversibility of ligand binding for this transcription factor. This in vitro assay could be used to assess future promising antagonists and complements existing cell-based reporter assays.

Exploiting Quorum Sensing Interfering Strategies in Gram-Negative Bacteria for the Enhancement of Environmental Applications

Quorum sensing (QS) is a widespread intercellular form of communication to coordinate physiological processes and cooperative activities of bacteria at the population level, and it depends on the production, secretion, and detection of small diffusible autoinducers, such as acyl-homoserine lactones (AHLs), auto-inducing oligo-peptides (AIPs) and autoinducer 2. In this review, the function of QS autoinducers of gram-negative bacteria in different aspects of wastewater treatment systems is examined. Based on research primarily performed over the past 10 years, QS involvement in the formation of biofilm and aerobic granules and changes of the microbial community and degradation/transformation pathways is discussed. In particular, the QS pathway in the role of bacterial infections and disease prevention in aquaculture is addressed. Interference of QS autoinducer-regulated pathways is considered potential treatment for a variety of environmentally related problems. This review is expected to serve as a stepping stone for further study and development strategies based on the mediation of QS-regulated pathways to enhance applications in both wastewater treatment systems and aquaculture.

Nitrite-Oxidizing Bacterium Nitrobacter winogradskyi Produces N-Acyl-Homoserine Lactone Autoinducers

Nitrobacter winogradskyi is a chemolithotrophic bacterium that plays a role in the nitrogen cycle by oxidizing nitrite to nitrate. Here, we demonstrate a functional N-acyl-homoserine lactone (acyl-HSL) synthase in this bacterium. The N. winogradskyi genome contains genes encoding a putative acyl-HSL autoinducer synthase (nwi0626, nwiI) and a putative acyl-HSL autoinducer receptor (nwi0627, nwiR) with amino acid sequences 38 to 78% identical to those in Rhodopseudomonas palustris and other Rhizobiales. Expression of nwiI and nwiR correlated with acyl-HSL production during culture. N. winogradskyi produces two distinct acyl-HSLs, N-decanoyl-l-homoserine lactone (C10-HSL) and a monounsaturated acyl-HSL (C10:1-HSL), in a cell-density- and growth phase-dependent manner, during batch and chemostat culture. The acyl-HSLs were detected by bioassay and identified by ultraperformance liquid chromatography with information-dependent acquisition mass spectrometry (UPLC-IDA-MS). The C=C bond in C10:1-HSL was confirmed by conversion into bromohydrin and detection by UPLC-IDA-MS.

Whole genome sequencing and analysis reveal insights into the genetic structure, diversity and evolutionary relatedness of luxI and luxR homologs in bacteria belonging to the Sphingomonadaceae family

Here we report the draft genomes and annotation of four N-acyl homoserine lactone (AHL)-producing members from the family Sphingomonadaceae. Comparative genomic analyses of 62 Sphingomonadaceae genomes were performed to gain insights into the distribution of the canonical luxI/R-type quorum sensing (QS) network within this family. Forty genomes contained at least one luxR homolog while the genome of Sphingobium yanoikuyae B1 contained seven Open Reading Frames (ORFs) that have significant homology to that of luxR. Thirty-three genomes contained at least one luxI homolog while the genomes of Sphingobium sp. SYK6, Sphingobium japonicum, and Sphingobium lactosutens contained four luxI. Using phylogenetic analysis, the sphingomonad LuxR homologs formed five distinct clades with two minor clades located near the plant associated bacteria (PAB) LuxR solo clade. This work for the first time shows that 13 Sphingobium and one Sphingomonas genome(s) contain three convergently oriented genes composed of two tandem luxR genes proximal to one luxI (luxR-luxR-luxI). Interestingly, luxI solos were identified in two Sphingobium species and may represent species that contribute to AHL-based QS system by contributing AHL molecules but are unable to perceive AHLs as signals. This work provides the most comprehensive description of the luxI/R circuitry and genome-based taxonomical description of the available sphingomonad genomes to date indicating that the presence of luxR solos and luxI solos are not an uncommon feature in members of the Sphingomonadaceae family.

Rhizobial homologs of the fatty acid transporter FadL facilitate perception of long-chain acyl-homoserine lactone signals

Quorum sensing (QS) using N-acyl homoserine lactones (AHLs) as signal molecules is a common strategy used by diverse Gram-negative bacteria. A widespread mechanism of AHL sensing involves binding of these molecules by cytosolic LuxR-type transcriptional regulators, which requires uptake of external AHLs. The outer membrane is supposed to be an efficient barrier for diffusion of long-chain AHLs. Here we report evidence that in Sinorhizobium meliloti, sensing of AHLs with acyl chains composed of 14 or more carbons is facilitated by the outer membrane protein FadLSm, a homolog of the Escherichia coli FadLEc long-chain fatty acid transporter. The effect of fadLSm on AHL sensing was more prominent for longer and more hydrophobic signal molecules. Using reporter gene fusions to QS target genes, we found that fadLSm increased AHL sensitivity and accelerated the course of QS. In contrast to FadLEc, FadLSm did not support uptake of oleic acid, but did contribute to growth on palmitoleic acid. FadLSm homologs from related symbiotic α-rhizobia and the plant pathogen Agrobacterium tumefaciens differed in their ability to facilitate long-chain AHL sensing or to support growth on oleic acid. FadLAt was found to be ineffective toward long-chain AHLs. We obtained evidence that the predicted extracellular loop 5 of FadLSm and further α-rhizobial FadL proteins contains determinants of specificity to long-chain AHLs. Replacement of a part of loop 5 by the corresponding region from α-rhizobial FadL proteins transferred sensitivity for long-chain AHLs to FadLAt.

Presence of acyl-homoserine lactones in 57 members of the Vibrionaceae family

AIMS

The aim of this study was to use a sensitive method to screen and quantify 57 Vibrionaceae strains for the production of acyl-homoserine lactones (AHLs) and map the resulting AHL profiles onto a host phylogeny.

METHODS AND RESULTS

We used a high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) protocol to measure AHLs in spent media after bacterial growth. First, the presence/absence of AHLs (qualitative analysis) was measured to choose internal standard for subsequent quantitative AHL measurements. We screened 57 strains from three genera (Aliivibrio, Photobacterium and Vibrio) of the same family (i.e. Vibrionaceae). Our results show that about half of the isolates produced multiple AHLs, typically at 25-5000 nmol l(-1) .

CONCLUSIONS

This work shows that production of AHL quorum sensing signals is found widespread among Vibrionaceae bacteria and that closely related strains typically produce similar AHL profiles.

SIGNIFICANCE AND IMPACT OF THE STUDY

The AHL detection protocol presented in this study can be applied to a broad range of bacterial samples and may contribute to a wider mapping of AHL production in bacteria, for example, in clinically relevant strains.

Unusual long-chain N-acyl homoserine lactone production by and presence of quorum quenching activity in bacterial isolates from diseased tilapia fish

Growth-dependent cell-cell communication termed quorum sensing is a key regulatory system in bacteria for controlling gene expression including virulence factors. In this study five potential bacterial pathogens including Bacillus sp. W2.2, Klebsiella sp. W4.2, Pseudomonas sp. W3 and W3.1 and Serratia sp. W2.3 were isolated from diseased Tilapia fish in Malaysia, supplied by the leading global fish supplier. Proteolytic activity assays confirmed that with the exception of Klebsiella sp. W4.2, all isolates showed distinct proteolytic activity. Furthermore Bacillus sp. W2.2 and Pseudomonas sp. strains W3 and W3.1 also displayed haemolytic activity. By using high resolution liquid chromatography mass spectrometry, we revealed the presence of unusually long-chain N-(3-oxohexadecanoyl)-homoserine lactone (3-oxo-C16-HSL) from Pseudomonas sp. W3.1 and N-dodecanoyl-homoserine lactone (C12-HSL) from Serratia sp. W2.3, respectively. Interestingly, Pseudomonas sp. W3.1 also produced a wide range of Pseudomonas quinolone signalling (PQS) molecules. Pseudomonas sp. W3 did not show any quorum sensing properties but possessed quorum quenching activity that inactivated AHLs. This study is the first documentation that shows unusual long-chain AHLs production in Serratia sp. and Pseudomonas sp. isolated from diseased fish and the latter also produce a wide range of PQS molecules.

The GtaR protein negatively regulates transcription of the gtaRI operon and modulates gene transfer agent (RcGTA) expression in Rhodobacter capsulatus

The gtaI gene of Rhodobacter capsulatus encodes an N-acyl-homoserine lactone (acyl-HSL) synthase. Immediately 5' of the gtaI gene is ORF rcc00328 that encodes a potential acyl-HSL receptor protein. A combination of genetic and biochemical approaches showed that rcc00328 (renamed gtaR) modulates the production of a genetic exchange element called the gene transfer agent (RcGTA), and regulates the transcription of gtaI. Although gtaI mutants exhibited decreased levels of RcGTA production, mutagenesis of gtaR did not, whereas a gtaR/gtaI double mutant produced wild-type levels of RcGTA. Because mutagenesis of gtaR suppressed the effect of the gtaI mutation, we suggest that the GtaR protein is a negative transcriptional regulator of RcGTA gene expression. We discovered that the gtaR and gtaI genes are co-transcribed, and also negatively regulated by the GtaR protein in the absence of acyl-HSL. A His-tagged GtaR protein was purified, and DNA-binding experiments revealed a binding site in the promoter region of the gtaRI operon. This GtaR protein did not bind to the RcGTA promoter region, and therefore modulation of RcGTA production appears to require at least one additional factor. We found that RcGTA production was stimulated by spent media from other species, and identified exogenous acyl-HSLs that induce RcGTA.

N-acyl homoserine lactone mediated interspecies interactions between A. baumannii and P. aeruginosa

Acinetobacter baumannii and Pseudomonas aeruginosa are pathogens capable of colonizing the same infection sites and employing N-acyl homoserine lactone (AHL) based quorum-sensing systems to co-ordinate biofilm formation. Hence, the effect of P. aeruginosa AHLs on biofilm formation by A. baumannii and vice versa were investigated using the biofilm impaired quorum sensing mutants, A. baumannii M2 (abaI::Km) and P. aeruginosa PAO-JP2. Complementing the mutants with heterologous, extracted and pure AHLs increased biofilm mass significantly. The surface area coverage and biovolume also increased significantly as observed by confocal scanning laser microscopy which corroborated scanning electron microscope analysis. Autoinducer synthase gene promoters of A. baumannii, P( abaI)-lacZ, and P. aeruginosa, P( lasI)-lacZ, were induced (p < 0.05) by heterologous AHLs. Growth of A. baumannii was not inhibited by pyocyanin of P. aeruginosa which may allow their co-existence and interaction in the clinical setting, thereby affecting the severity of combined infections and therapeutic measures to control them.